The purpose of this research is to develop a Radionuclide Therapy Treatment Planning (RTTP(C)) software package. This software will calculate 3-D, voxel-based absorbed dose distributions, from a CT scan and computer-simulated radiopharmaceutical biokinetics, using two Monte Carlo radiation transport techniques. Using a voxel-based geometry (i.e., CT scan) for combined electron/photon transport calculations, 3-D energy deposition distributions will be calculated. Simulated single photon emission computed tomography (SPECT) images will provide the data for calculating voxel-based source strength (radioactivity) distributions. Convolution of the 3-D activity distribution with the energy deposition pattern, also in a 3-D matrix format, will provide a patient-specific estimate of the 3-D absorbed dose distribution. This dose distribution can then be redisplayed as isodose contours or absorbed dose histograms for normal organs and lesions. In Phase II, the voxel-based source strength distributions will be measured directly using SPECT imaging of both phantoms and patients. SPECT has the capability to measure, in vivo, nonuniform activity depositions. Image fusion provides an elegant method of interpreting the nonuniform absorbed dose distributions. The dose-volume histograms can be used in quantitatively choosing among treatment planning options and in the determination of tumor control probabilities (TCP) and normal tissue complication probabilities (NTCP). PROPOSED COMMERCIAL APPLICATION: The availability of RTTP(C) software could accelerate clinic trials, reduce the size of dose escalation studies, and otherwise speed regulatory review/approval of therapy radiopharmaceuticals. Use of this software will optimize selection of alternative treatment plans, reduce potential therapeutic errors, reduce cancer therapy costs, increase physician acceptance of radionuclide therapies, and improve cancer patient quality of life as more radionuclide therapies are brought to market.